Ofdm & Wi Max

OFDM and Wireless Interoperability for Microwave Access (WiMAX)

1Seminar on Signal Processing in

Wireless Communications 2006/07

OFDM

and

WiMAX

Sergio González

15th January 2007




-What is OFDM?

Orthogonal Frequency-Division Multiplexing

- Digital modulation scheme based upon the principle of frequency-division

multiplexing (FDM)

- A high rate bit stream is divided into several lower rate parallel bit streams

- The available spectrum is divided into sub-channels, and each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example PSK, QAM, etc




- ¿Orthogonality..?

Definition of this mathematical concept:

Two functions f(x) and g(x) are orthogonal if their inner product is zero:

This inner product is linear so:

For OFDM schemes a group of orthogonal carriers is needed, so this generatea base (S) of the signal space:

where M=(0,..,n), for n different subcarriers

All the element of S are orthogonal to each other:

if normalizedfunctions

( ) ( ) ( ) ( )xpxukxpxuk ,, =×

( ) ( ) ( ) ( ) 0*, =××= ò¥=

¥-dxxgxfxgxf

x

( ){ } Mkk tS ÎY=

=YY ji ,Y




If the symbol length is T, sinusoidal signals differing in frequency by 1/T will be orthogonal over the period T:

So this functions can be the seed of our orthogonal function space base

and its sprectral representation would be:

( )[ ]

( )0,

12

21

22 0

00

0 =úû

ùêë

é×=

+P-+

P+P

P

ò dteeeet

Tfj

T

tfj

T

tT

fjtfj

t

t

( )

Nk

tT

Kfj

eSÎ

+P-

þýü

îíì

=02




One single OFDM symbol s(t) would be:




For a invariant linear channel, the received signal would be:

So instead of n filters, n correlators are needed. Initial pilot tones are needed for channel estimation




-Subcarrier demodulation in OFDM

A QPSK, QAM, BPSK scheme will be normally used.

Mathematically this means, that the subcarrier is multiplied by the mapping symbol ofthe I-Q modulation diagram in the transmitter system, what would be a complexnumber. The following analysis would be for a single carrier impinging the receiver. Dueto its linear properties it can be extensible for n subcarrier :

-R(t) is the received signal.

-Ψ(t) is the transmitted carrier (orthonormal in this case) and a k is the

mapped symbol.

-n(t) is the noise contriubution and K is the channel linear contribution

So the receiver system must be able to make n inner product operations and then anestimation of the recieved a k vector.

( ) ( )

( ) ( ) ( ) åååå==

^==

Y+×=Y++YY×=Y=

+Y××=

==

n

i

ik

n

i

i

n

i

ipk

n

i

i

pk

tnaKtntntaKtRty

tntaKtR

0000

),(),()(,,

)(




OFDM Advantages:* Robust against narrow-band co-channel interference

* Robust against Intersymbol interference (ISI) and fading caused by multipath propagation

* High spectral efficiency

* Easy implementation using FFT and IFFT algorithms for modulation and correlation operations

* Low sensitivity to time synchronization errors

* Tuned sub-channel receiver filters are not required (unlike conventional FDM)

But also some disavantages:# Sensitive to Doppler shift

# Sensitive to frequency synchronization problems.

# Inefficient transmitter power consumption, due to linear power amplifier requirement




The OFDM sensibility to multipath channel has to be thwarted by some mechanism for Wirelesssystems

Cyclic PrefixSo as to avoid power loss from echoes a copy of the end is appended at the beginning

The length of the Guard Interval has to be chosen to be longer than the last significant echo, so once this prefixed is removed we only have the modulated and faded subcarriers




Some OFDM applications are:

• ADSL

• WAN and LAN applications as IEEE 802.11a/g and Wimax

• Digital radio

• DVB-T

• Candidate for future 4G communication systems




Fixed Broadband Wireless Access

and

Wireless Interoperability forMicrowave Access (WiMAX)




Some introductory definitions:

Broadband “having instantaneous bandwidth greater than

around 1 MHz and supporting data rates greater than about 1.5 Mbit/s”.(IEEE 802.16-2004)

WiMAX is an Air Interface for Fixed Broadband Wireless Access Systems, also known as the IEEE WirelessMAN air interface, base concept of IEEE 802.16x family




What is Wimax?

Official definition:

“WiMAX is not a technology, but rather a certification mark, or 'stamp of approval' given to equipment that meets certain conformity and interoperability tests for the IEEE 802.16 family of standards……. “ [Wimax Forum]

Wimax is an industry group

A group of services providers, manufacturers, and related companies that have joined together to promote the family of technologies based upon the IEEE standard 802.16 in the Wimax Forum

Are IEEE 802.16 and WiMAX synonymous?

IEEE 802.16 is a wireless communication standard and Wimax a trademark. So technically, no – but yes from a common usage perspective and for most all discussions. Just as 802.11 and WiFi are commonly used interchangeably.




What is the IEEE 802.16 Standard?

Point-to-Multipoint (PMP) broadband wireless access standard forsystems in the frequency ranges 10 – 66 GHz and under 11 GHz designed for “last mile” applications.

IEEE 802.16 extensions:

•802.16a Use the licensed and license-exempt frequencies from 2 to

11Ghz (support Mesh-Network)

•802.16b Increase spectrum to 5 and 6GHz –Provide QoS (for real-time voice and video service)

•802.16c Represents a 10 to 66GHz system profile

•802.16-2004 Project coclusion for fixed system, and withdrawal of all

previous amendments.

•802.16d Improvement and fixes for 802.16a

•802.16e Mobile WiMAX (enable high-speed signal handoffs necessary for communications with users moving at vehicular speeds)







Wimax evolution




World available spectrum




Comparing IEEE 802 with other Wireless technologies




WiMAX vs. WIFI (OFDM)

TDDFDD/TDDDuplex

20 Mhz

Turbo 40 Mhz

From 1.5 MHZ to 28MHZ

Bandwidth

48Pilot carriers

53200/aprox.1700Data carriers

64256/2048Total carriers

WIFIWiMAXParameter




WiMAX vs. WIFI (OFDM)

NoYes

MIMOSmart antennas

NoYesSubchannels

FixedConfigurableGuard

Interval

BPSK,QPSK, 16QAM, 64QAM

BPSK,QPSK, 16QAM, 64QAM

Modulation

schemes

WIFIWiMAXParameter




Wireless systems comparison




WiMAX AWG application classes




Some application escenario




- For further information seehttp://www.wimaxforum.orghttp://www.intel.com

- Remarks or questions…..

Ofdm & Wi Max

Technology